Are you saying you have a power injector and from it you have one cable going from the PoE out to the rb411 PoE in? Then another cable from the PoE LAN port to the switch?
You say there are no other cables connected to the switch or the rb411 correct?
Can you give more detail about what switch you have and if there really is anything else connect to it or the rb411.
What your saying doesn't make much sense.. you could have a bad cable or faulty power injector. As mentioned in another post it sounds like you have a loop in the network but... who knows. Take a picture, draw something up, whatever it takes to get your point across better.